Coinbase·Software Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

Coinbase software engineering interview with a deep systems question about building an iterator library from scratch. Single question but it covered a lot of ground, from lazy combinators to invalidation semantics to unit tests. Felt more like a take-home that got crammed into a live session.

Questions Asked (1)

Design and implement a full iterator library supporting range(start, end, step) with forward and backward traversal, inclusive and exclusive endpoints, negative steps, and overflow-safe termination. Include lazy combinators (map, filter, zip, take, drop, chain) each using O(1) space, a PeekableIterator with peek() and hasNext(), and discuss whether reset()/rewind() is feasible without materializing the sequence. Also define iterator invalidation rules when the underlying collection mutates, and write unit tests for edge cases like zero step, empty ranges, very large ranges, and negative steps.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining the iterator interface, then design the core range iterator with overflow-safe termination and support for negative steps. Implement lazy combinators using O(1) space and a PeekableIterator, and discuss reset/rewind feasibility and invalidation rules. Conclude with edge-case tests and trade-offs.

Pro tip: Emphasize overflow safety by using saturating arithmetic or checked operations, and explain how lazy evaluation avoids materialization, which is crucial for large ranges. Also, proactively discuss iterator invalidation semantics to show depth.

1. Clarify requirements and define interfaces

Ask clarifying questions about inclusivity, step sign, overflow behavior, and mutation semantics. Define the Iterator interface with hasNext() and next(), and the PeekableIterator with peek().

2. Design the range iterator

Implement range(start, end, step) with forward/backward traversal, inclusive/exclusive endpoints, and negative steps. Use overflow-safe termination by checking if the next value would exceed the end in the direction of the step.

3. Implement lazy combinators

Create map, filter, zip, take, drop, and chain as lazy iterators that wrap the source and compute values on demand, ensuring O(1) space by not storing intermediate sequences.

4. Add PeekableIterator and discuss reset/rewind

Implement PeekableIterator with a one-element buffer. Discuss that reset/rewind is feasible only if the underlying iterator is re-iterable or if the sequence is materialized; otherwise, it's not possible without extra space.

5. Define invalidation rules and write tests

Specify that iterators are invalidated if the underlying collection is structurally modified (e.g., add/remove) during iteration, similar to Java's ConcurrentModificationException. Write unit tests for zero step, empty ranges, large ranges, negative steps, and mutation scenarios.

Key Points to Mention

Overflow-safe termination: use saturating arithmetic or check if next value would exceed end without overflow.
Lazy evaluation with O(1) space: combinators like map and filter do not store elements; they process on demand.
PeekableIterator: buffer one element to support peek() and hasNext() without advancing.
Reset/rewind feasibility: only possible if the source is re-iterable or if the sequence is materialized; otherwise, not feasible without O(n) space.
Iterator invalidation: fail-fast behavior on structural modification, with clear rules (e.g., throw ConcurrentModificationException).
Edge cases: zero step (throw IllegalArgumentException), empty ranges (handle gracefully), large ranges (avoid overflow), negative steps (reverse traversal).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.